Tuning low-inductance coils at low frequencies

Abstract

A method and apparatus for tuning and matching extremely small sample coils with very low inductance for use in magnetic resonance experiments conducted at low frequencies. A circuit is disclosed that is appropriate for performing measurements in fields where magnetic resonance is beneficially utilized. The circuit has a microcoil, an adjustable tuning capacitance, and added inductance in the form of a tuning inductor. The microcoil is an electrical coil having an inductance of about 25 nanohenries (nH) or less. Because additional inductance is purposefully added, the capacitance required for resonance and apparatus function is proportionally and helpfully reduced. The apparatus and method permit the resonant circuit and the magnet to be made extremely small, which is crucial for new applications in portable magnetic resonance imaging, for example.

1. A method for obtaining magnetic resonance signals from a microcoil at low frequency comprising the steps of:
 connecting the microcoil in series with a second coil, whereby the microcoil is an effective magnetic resonance transmitter or receiver coil; 
 forming a resonant circuit of the coils with a capacitor whose capacitance is determined primarily by the inductance of the second coil; 
 permitting the resonant circuit to resonate at low frequency, with alternating current at a resonance wavelength; 
 locating the microcoil remotely from the second coil and capacitor; and 
 connecting electrically the microcoil in parallel with the second coil by means of a transmission line having a length that is an odd multiple of a length corresponding substantially to one-fourth of the resonance wavelength; 
 whereby the remotely located microcoil presents, in the resonant circuit, as a large impedance in parallel to the second coil. 
 
   
   
     2. A method according to  claim 1 , wherein the step of connecting the microcoil in parallel with the second coil comprises connecting the transmission line to a partial segment of the second coil. 
   
   
     3. A method according to  claim 1 , wherein the step of connecting the microcoil in parallel with the second coil comprises connecting an impedance transformer to the transmission line intermediate to the microcoil and the second coil. 
   
   
     4. A method according to  claim 1  wherein the step of connecting the microcoil in series with a second coil comprises connecting electrically a plurality of microcoils in series, and connecting said plurality of microcoils in series with the second coil. 
   
   
     5. A method according to  claim 1  wherein the step of connecting the microcoil in series with a second coil comprises connecting electrically a plurality of microcoils in parallel, and connecting said plurality of microcoils in series with the second coil. 
   
   
     6. A method for obtaining magnetic resonance signals from a microcoil at low frequency, comprising the steps of:
 connecting the microcoil in series with a tuning coil having an inductance at least ten times larger than the inductance of the microcoil, whereby the microcoil is an effective magnetic resonance transmitter or receiver coil and contributes no substantial inductance needed for resonance; 
 forming a resonant circuit of the coils with a capacitor whose capacitance is determined primarily by the inductance of the tuning coil and; 
 permitting the resonant circuit to resonate at low frequency, with alternating current at a resonance wavelength. 
 
   
   
     7. A method according to  claim 6  wherein the step of forming a resonant circuit comprises forming a parallel resonant circuit wherein the microcoil contributes no substantial inductance needed for parallel resonance. 
   
   
     8. A method according to  claim 7  wherein the step of connecting the microcoil in series with a tuning coil comprises connecting electrically a plurality of microcoils together, and connecting said plurality of interconnected microcoils in series with the tuning coil. 
   
   
     9. A method according to  claim 7  further comprising the steps of:
 locating the microcoil remotely from tuning coil and capacitor; and 
 connecting electrically the microcoil in parallel with the tuning coil by means of a transmission line having a length that is an odd multiple of a length corresponding substantially to one-fourth of the resonance wavelength; 
 whereby the remotely located microcoil presents, in the resonant circuit, as a large impedance in parallel to the tuning coil. 
 
   
   
     10. A method according to  claim 9 , wherein the step of connecting the microcoil in parallel with the tuning coil comprises connecting the transmission line to a partial segment of the tuning coil. 
   
   
     11. A method according to  claim 6  wherein the step of forming a resonant circuit comprises forming a series resonant circuit wherein the microcoil contributes no substantial inductance needed for series resonance. 
   
   
     12. A method according to  claim 11  wherein the step of connecting the microcoil in series with a tuning coil comprises connecting electrically a plurality of microcoils together, and connecting said plurality of interconnected microcoils in series with the tuning coil. 
   
   
     13. A method according to  claim 11  further comprising the steps of:
 locating the microcoil remotely from tuning coil and capacitor; and 
 connecting electrically the microcoil in parallel with the tuning coil by means of a transmission line having a length that is an odd multiple of a length corresponding substantially to one-fourth of the resonance wavelength; 
 whereby the remotely located microcoil presents, in the resonant circuit, as a large impedance in parallel to the tuning coil. 
 
   
   
     14. A method according to  claim 13 , wherein the step of connecting the microcoil in parallel with the tuning coil comprises connecting the transmission line to a partial segment of the tuning coil. 
   
   
     15. A method according to  claim 13 , wherein the step of connecting the microcoil in parallel with the tuning coil comprises connecting an impedance transformer to the transmission line intermediate to the microcoil and the tuning coil.